The present invention relates generally to the production of potato chips and other food products prepared by deep-fat frying, and is particularly concerned with a continuous frying method for making potato chips which are similar in taste and texture to those produced by the slow-cooked batch or kettle process. In a particular embodiment, the invention relates to a process whereby reduced-oil potato chips are made.
Large scale commercial production of potato chips is usually carried out in a continuous frying apparatus in about 3 to about 3.5 minutes at temperatures between about 300.degree. F (about 149.degree. C) and about 360.degree. F (about 182.degree. C). If all or a portion of the frying process is carried out below about 290.degree. F (about 143.degree. C), for a period of about 7 to about 11 minutes, a distinctly different chip results. The chip produced by the lower temperature process is harder and crunchier, and its surface takes on a more bubbly appearance. In addition, the flavor is somewhat blander than that of a regular potato chip, and there is a more pronounced oiliness to the mouth feel. These characteristics are deemed very desirable by many producers and consumers of potato chips. Since the original method of making this type of chip was by the batch, in relatively small vessels of oil heated directly over a flame, the chips were termed "kettle style".
Unfortunately, the batch process is very labor intensive and requires manual agitation as with a rake to avoid clumping and to attain dunking for evenness of frying. Furthermore, it can produce only small quantities of finished product, typically between 50 and 200 pounds per hour per kettle. By contrast, large scale potato chip producers typically employ continuous fryers capable of producing on the order of 1,000 to 5,000 pounds per hour of finished product The kettle or batch process is therefore uneconomical for large scale manufacturers.
For a number of reasons, it has been problematic to obtain desirable qualities of kettle style potato chips in chips produced by conventional continuous fryers. In a typical continuous fryer, raw potato slices are introduced at one end of an elongated vessel or trough containing heated frying oil. While immersed in the oil, the potato slices are carried toward the opposite end of the vessel by mechanical conveying devices or more commonly by the velocity of the oil itself. Frying takes place as the water in the potato slices is driven out and replaced with oil, and the completely fried chips are removed when they reach the end of the vessel. In order to maximize the production rate and reduce sticking between the potato slices, high temperatures are utilized and the residence time of the slices in the frying oil can be held to a minimum. These conditions result in a chip with a taste and texture different from that of kettle style chips.
Continuous fryers are also characterized by a temperature gradient in the oil bath which decreases continually from a maximum value at the entrance end of the fryer, where the oil is typically admitted after being heated by a heat exchanger or other type of heat source, and the exit end where the oil is withdrawn for reheating and recirculation. By contrast, the time-temperature profile in the kettle or batch process is more complex, initially decreasing as the relatively large amount of water in the raw potato slices absorbs heat from the oil and then increasing after most of the water has been vaporized and driven out. This is another factor which accounts for the differences between potato chips produced by the kettle process and those produced by continuous fryers.
A particular type of kettle style chips has been produced which have an especially crisp texture and which retain more intrinsic potato flavor after frying. These chips are prepared by frying uncooked potato slices for a particularly long period of time at relatively low temperatures without previously washing the slices to remove surface starch and other surface components. However, the unwashed slices are exceeding sticky, particularly when fried at low temperatures. Conventional continuous fryers typically provide neither enough vertical frying area to allow for longer frying times nor sufficient agitation in the appropriate region to counteract agglomeration of sticky slices. In fact, agglomeration of 85% by weight of fried potato chips is not uncommon. Agglomeration above about 2% by weight, however, is unacceptable because agglomerated clusters of potato chips are unable to fry out sufficiently.